Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant, which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier media include water, mixtures of water and organic solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc.
Materials used in inkjet printing inks must have the correct properties to provide an ink which is stable, possesses good printing properties and provides an image with good color, sharpness and image stability. Many dyes are known and used in inkjet printing inks. Many have some or most of these desirable properties but it is very difficult to find a dye which possesses all of the above attributes. Ink jet inks generally contain a dye that is soluble in an ink vehicle such as water or a mixture composed of water and a known water soluble or water miscible organic solvent. Typically the dyes are chosen from acid, direct and reactive dyestuffs developed for the dyeing of natural fibers such as paper, wool and cotton. Water solubility of these dyes is due to the incorporation of negatively charged substituent groups such as sulfo or carboxy. Dyes are degraded by ozone and light and their stability with regard to these two agents can differ depending on media and ink composition. There is a great need to develop dye-based inks which have high optical densities on receivers and also superior lightfastness and colorfastness when printed on different types of media, in particular, fast drying or porous media.
The yellow dyes currently in commercial aqueous ink formulations are less than optimal in one property or another. They are used because they achieve an acceptable but not superior balance of features. Some examples of such dyes are Direct Yellow 132 (CAS 10114-86-0) and Direct Yellow 86 (CAS 50295-42-3), which have good stability toward ozone and light but have less than optimal hue, Acid Yellow 23 (CAS 1934-21-0) and Acid Yellow 17 (CAS 6359-98-4) which have good hue and ozone fastness, but poor light stability. U.S. Pat. Nos. 6,468,338 and 6,464,767 disclose water-soluble azoindole dyes for use in ink jet printing including dyes derived from diazotizable heteroaromatic amines. However, there is a problem with some of these dyes in that they do not have sufficient stability to atmospheric ozone. US2001/0029869 discloses a large number of heterocyclic azo dyes, selected from a number of coupling classes and heterocyclic diazo moieties. These dyes have a range of stability, due to the different types of heterocyclic azo moieties which are part of their structures; some have poor, others have good stability to light and ozone, particularly on porous, fast-drying ink jet media. This property on porous media is important because such fast drying media is becoming more and more popular and, while most dyes are stable on gel-based or swellable media, porous media are more stringent in their demand for a high level of stability. The range of hue provided by these dyes also varies because there is such a wide variety of structure space covered by application US2001/0029869. However, the cited reference gives no guidance to one skilled in the art who wishes to select such dyes, or those moieties in the structure of the dyes, that provide the best performance in ozone and light stability, with the most desirable hue, when printed on porous ink jet media